2173. Shelves, except such as are placed near the ground, or almost close under the upper angle of the roof, are extremely injurious to the vegetation going forward in the body of the house, by the exclusion of light. This consideration, therefore, must be kept in view in placing them: in some cases they are inadmissible, as in conservatories; in others, as in propagating-houses, the light they exclude can better be spared, than in fruiting or flowering departments. For forcing strawberries, they may be introduced under the roof in vine and peach houses, and removed when their shade proves injurious, &c. The ordinary form is that of a flat board; but an improvement consists in nailing two fillets along its edges, and covering the board with a thin layer of small gravel or scoriï¾µ. This preserves a cool genial moisture, which keeps the earthen pot moist, and lessens the effect on the earth of alternate dryings and waterings; and it also admits the more ready escape of water from the orifices in the bottoms of the pots. Some, in the case of forcing strawberries and kidneybeans, have the fillets or ledges of the shelves so high as to contain 2 or 3 inches of water, by which means whole rows of pots can be inundated at one operation; but this is too indiscriminate an application of a material on which so much in the growth of plants depends.